The principle of a floor with co-operation between wood and concrete is known. Its strong profile is a T-shaped profile whose bottom portion is a beam of wood and whose top portion is a slab of concrete which is, generally in compression. Such a slab is cast onto shuttering which may optionally be retained or removed, and which is disposed on the beams or between the beams.
Co-operation between the wood and the concrete is provided by metal connectors whose stiffness and retention both in the wood and in the concrete prevent any relative displacement between the component parts in the span direction, i.e. in the longitudinal direction of the beams. The distribution of the connectors is similar to the distribution of stirrup-shaped binding wires in a reinforced concrete beam.
A distribution trellis made of steel is embedded in the concrete and provides the slab with strength against transverse bending and against puncture.
For strength of materials purposes, composite beams constituted by this T-shaped profile are generally simply supported at their ends. This is the simplest case and is considered below by way of example, but it should be understood that if the beams are locally encastre or bear on intermediate supports, then forces will be locally inverted. In the present simple case, composite beams operate in simple bending. Their neutral fiber is preferably situated in the vicinity of nonremovable shuttering and the dimensions of the wood and the concrete are chosen accordingly. The concrete works in compression and the wood works in traction along the longitudinal direction, and the connectors are subjected to internal shear forces which are exerted between the wood and the concrete in the same longitudinal direction.
In a first prior floor of this type, the connectors are constituted by vertical nails which are partially engaged in the top faces of the joists through the shuttering before the slab is cast. The head and the top portion of each nail is embedded in the concrete during casting. Such a floor is described in an article by Godycki et al entitled "Verbunddecke aus Holzrippen unde Betonplatte" (Bauingenieur 59 (1984) 477-483, Springer-Verlag, West Germany). Such connectors suffer from the drawback of their middle and bottom lengths bending easily under the effects of the above-mentioned internal longitudinal forces. They then bend in the wood. This connector bending gives rise to deformation of the floor and to a reduction in its strength.
In a second prior floor, the connectors are likewise made of metal, and they are rigid. They are constituted by connection plates made of metal sheet and extending vertically and longitudinally in contact with the two sides of each beam. Pointed transverse horizontal nailing teeth may be formed by cutting and horizontally folding various zones of each such plate. These teeth penetrate into the side of the adjacent beam in order to fix the plate during an operation which may be referred to as "nailing". However, this operation may also be performed by means of ordinary nails extending horizontally and passing through the plates. The top portion of each plate projects above the beam and is cut to form a series of vertical pointed connection teeth which pass through the shuttering when the shuttering is put into place on the beams and which are subsequently embedded in the concrete of the slab.
One such second floor is mentioned without being described in detail in European patent document EP-Al-0104629 (Poutanen, Tuomo Tapani).
This second prior floor has the particular advantage that the shape of each connector ensures that the assembly is very rigid against bending forces constituted by the above-mentioned internal longitudinal forces. However, it suffers from various drawbacks including those mentioned below.
(1) If the above-mentioned nailing operation is performed using nailing teeth prior formed by cutting and folding various zones of the plate, then the nailing operation requires a press to be used which is capable of exerting a considerable force.
If the structure is a new building then the force is relatively easily applied by means of a large press permanently disposed in a workshop. However storing and handling beams becomes difficult after this operation has been performed because of the poor stiffness of the connection teeth and the plates in a transverse direction. However, if beams that are already in place are being used, it is generally difficult to bring such a large press into contact with the beams. Finally, if the nailing operation is performed with ordinary nails, these nails may bend in the wood.
(2) The connection teeth constitute groups within which the teeth follow one another longitudinally at small intervals. When the slab is being cast, the concrete aggregate has difficulty in inserting itself between pairs of successive teeth, thereby weakening the concrete in a zone where it is highly stressed by the connector.
(3) The connection between the plate and the beam cannot be completely rigid. If the nailing teeth are so formed when the connector is made as to occupy transverse vertical planes by folding the metal sheet about vertical fold lines, then longitudinal forces in operation can easily fold them about such vertical lines. If the teeth are formed in horizontal planes by folding about horizontal longitudinal lines, then the teeth present only a very small thrust area for longitudinal thrust inside the wood, i.e. they present an area corresponding to the thickness of the sheet, and as a result longitudinal forces applied in service can easily thrust the teeth through the wood.
(4) If it is unacceptable for the slab shuttering to be pierced by the connection teeth when the shuttering is put into place, and if it is preferred for the shuttering to be constituted by sheets of shuttering each placed on the edges of two adjacent beams, then connection plates which are nailed to the sides of the beams and which project locally above the beams get in the way when placing the shuttering plates on the beams.
(5) The bottom portions of the plates remain visible and spoil the uniform appearance of the wood.
(6) Since the plates are apparent and penetrate into the sides of the beams, the connections provided thereby are exposed to the high temperatures which can occur during a fire. They are thus unsatisfactory from the safety point of view.
The present invention seeks to provide, cheaply, a floor having co-operation between wood and concrete and which is of increased strength. More particularly, the invention seeks to provide connectors and to put them into operation in such a floor in a manner which is simple and effective and which avoids the above-mentioned drawbacks.